Discharge lamps operate with current limited by a ballast. There are various types of ballasts. At one end is the simple conventional ballast inductor used in sodium lights at powerline frequency. At the other end are the electronic ballasts, in which semiconductors are used to control lamp current. The control elements in electronic ballasts operate at a much higher frequency than is the case with conventional ballasts, and the output frequency of electronic ballasts may be controlled independently from the current-regulating stage of the ballast.
Electronic ballasts are commonly used for location-lighting in filming environments, where the current regulator operates at a high frequency, say 25 kHz, and the output frequency of the ballast is low, say 100 Hz. This is achieved by passing the regulated current through a low-frequency inverter. Such lamps typically operate at power levels anywhere between 200 W and 18 KW. More information on the difference between conventional magnetic ballasts and electronic ballasts can be found in Chapter 5 of “Philips' Medium Source High Intensity Discharge Lamps: Information for Luminaire and Ballast Manufacturers”, published by Philips Lighting BV, Belgium, March 2000.
Some electronic ballasts drive the lamp of a lamp system at high frequency. This gives the advantage of a simple power stage, because a low-frequency output inverter is not required. However, operation in this manner is generally limited to low-power lamps because of the problem of “acoustic resonance” (see Section 5.2 of the above Philips reference). In order to avoid acoustic resonances, the driving frequency must be above the highest resonant frequency of the lamp. It is lamp systems operating at these higher frequencies that are the subject of this invention.
FIG. 1, which is taken from Section 4.1 of the above Philips' reference, illustrates the different phases from the moment of switching on the supply power to stable lamp operation for a HID lamp. A successful ignition operation proceeds through all of the voltage-versus-time phases illustrated in FIG. 1, while an unsuccessful ignition results when the process stops in one of those phases. The time scale in FIG. 1 is logarithmic so as to better illustrate the times involved.
The phases in FIG. 1 are; (a) ignition delay, (b) breakdown, (c) take-over, (d) glow, (e) glow-to-arc transition, (f) run-up (arc) phase, and (g) stable operation. The stable operation phase after ignition corresponds to that in which “steady-state voltage” and “steady-state current” operate, as those terms are further used in this document.
FIG. 2, which is taken from Section 5.4 of the above philips' reference, illustrates one form of ignitor circuit that may be used for ignition of a high-intensity discharge lamp. The ignitor circuit in FIG. 2 includes a time switch 15 that provides temporary tapping of the power-line voltage, a transformer Tr1 for raising the tapped voltage, and a primary winding P1 of a transformer Tr2 for introducing the raised voltage into the steady-state circuitry of a lamp 17; the secondary windings S1 and S2 of the transformer Tr2 are serially-connected to the lamp 17 and are phase-adding windings. (The term “phase-adding” as used in this document refers to windings that are wound to provide voltage amplification on an output circuit.) A single secondary S1 optionally may be used, as represented by the dashed line in FIG. 2. The difficulty with the type of ignitor circuit shown in FIG. 2 is that the large number of turns on S1 and S2 result in a value of inductance which is too large to allow the necessary amount of lamp current to flow when operating at high frequency. Additionally, high flux levels are induced which would cause overheating of the magnetic core. As will be discussed below, the subject invention seeks to improve on this arrangement.
Among other uses, the subject invention has particular application to HID lamps that provide location lighting in the film industry, although it also has application to other forms of entertainment, such as rock shows, theatre shows, etc. Reliable ignition of location-lighting lamps is of crucial importance for location film shoots, which cannot proceed without proper and sufficient lighting; any delay resulting from lighting problems has a significant cost implication. When HID lamps are hot, they are difficult to ignite since the required ignition voltage is increased; this is the so-called “hot-restrike” condition. There is reference to such hot-restrike ignition capability in the patent literature. For instance, WIPO Patent Publication WO 97/43875 refers to a “hot restrike condition” in regard to a metal halide lamp. However, the voltage required for hot-restrike ignition at the lamp in that reference is considerably below that required by a metal halide lamp used for location lighting in the film industry, where lamps typically operate at a power level 200 W and above.